When a gas leak occurs, a slight temperature change occurs in a region where the leaking gas is drifting. As a technique for detecting a gas by using this principle, gas detection using infrared images is known. Since this enables real-time display of a moving image indicating how a gas is leaking (in other words, a region where the leaking gas is drifting), it is possible to intuitively determine a range of the gas leak.
As gas detection using infrared images, for example, Patent Literature 1 discloses a gas leak detection device including an infrared camera that shoots a region to be inspected and an image processing unit that processes infrared images shot by the infrared camera, and the image processing unit includes a fluctuation extraction unit that extracts dynamic fluctuation caused by a gas leak from a plurality of infrared images arranged on a time-series basis.
When a gas leak occurs, a temperature change caused by the leaking gas is small (e.g., 0.5° C.). For an outdoor object to be monitored for a gas leak (e.g., a place where gas transport pipes are connected to each other), when a cloud moves to block sunlight or a cloud blocking sunlight moves, the temperature of a background to be monitored changes more sharply and greatly than the temperature change caused by the leaking gas (e.g., 4° C.).
When the gas leak and the background temperature change occur in parallel, in a region where the leaking gas is drifting, the temperature change caused by the gas leak will coincide with the background temperature change. The inventors have found out that in such a case, if the background temperature change is larger than the temperature change caused by the leaking gas, unless the background temperature change is taken into consideration, it is not possible to display as infrared images how the gas is leaking, that is, it is difficult to detect the gas.